A polyurethane foam is prepared by foaming and setting a foamable composition comprising main components of an active hydrogen compound having at least two active hydrogen atoms and an isocyanate compound having at least two isocyanate groups. The foamable composition can comprise a blowing agent, a catalyst, a foam regulator etc. together with the two main components. Sometime, it further comprises additional components such as flameproofing agent, a coloring agent, a stabilizer, a filler etc.
The foamable composition usually begins to cause foaming within at least several tens seconds after the blending and completes the foaming within several to ten and several minutes after the blending. Tackiness of the surface is lost to complete the setting at certain time after completing the foaming.
In the technical field, a period from the completion of the blending to the time just before blowing a white creamy liquid of the foamable composition in the initiation of foaming is called as a cream time, and a period for blowing the creamy liquid to grow a foam and to complete the blowing is called as rise time and a period for setting the surface of the foam after the rise is called as set time.
It is relatively difficult to form a uniform thin foamable composition layer by spreading a foamable composition obtained by mixing components from a feeding outlet to a surface which is relatively moved to the feeding outlet.
These surfaces for relatively moving include slab belt surface, a mold inner wall and a plate surface. The feeding outlet of a mixer is usually in a form of holes. Therefore, even though a foamable composition is fed on a surface which is relatively moved to the feeding outlet, the foamable composition is placed in the form of strips on the surface. In the conventional slab formation of a soft polyurethane foam, such process has been employed. Since the foamable composition has low viscosity, the foamable composition is spread on the surface of the slab belt by its dead weight and the three-dimentional blowing is resulted and the foamable composition is expanded to required width since the blowing rate is large. When the width of the slab belt is large, the feeding outlet is reciprocated in the transversal direction of the slab belt so as to place the foamable composition in zig-zag on the surface of the slab belt. It is difficult to employ said method when a foamable composition for a hard polyurethane foam has high viscosity or a foamable composition should be placed in thin thickness for forming a thin polyurethane foam or a blowing rate of a foamable composition is low.
A method for distributing a foamable compositon on a whole surface which is relatively moved to a linear feeding outlet for the foamable composition which is arranged to a perpendicular direction to the moving direction of the relative movement has been known. For example, in the specification of U.S. Pat. No. 3,927,162, a method for feeding a foamable composition on a whole inner surface of a moving mold from a sector feeding outlet is described. However, in this method, in order to discharge uniformly a foamable composition from all part of the linear feeding outlet, a pressure being higher than certain degree is required. When the pressure is lower than this degree, a discharge from the top of the feeding outlet is difficult. When a width of the feeding outlet is narrower without varying its length to reduce a feeding rate of the foamable composition, the clogging of the foamable composition is easily caused to be difficult feeding uniformly. A foamable composition is foamed and set for a short time, whereby the clogging of the feeding outlet is caused by the foamable composition set near the linear feeding outlet and the uniform feeding of the foamable composition from all position of the linear feeding outlet is difficult. This phenomenon is especially caused when the width of the feeding outlet is narrow and the feeding rate of the foamable composition is low.
A method for spraying a foamable composition by a spray has been known. However, a nozzle of the spray has holes of the feeding outlet. Even though particles of the foamable composition is sprayed in spread, the sprayed composition is placed in a circular form. Therefore, even though the surface is relatively moved, the foamable composition is placed in a broad linear form. Even though the spray nozzle is reciprocated in the transversal direction on the surface which is relatively moved to overlap edges of the broad linear form of the foamable composition on the surface, it is difficult to give uniform quantity of the foamable composition at the overlap-sprayed place and non-overlap-sprayed place. Moreover, in order to spray a foamable composition, a compressing means such as use of compressed air is required and a spraying of a foamable composition having high viscosity is difficult and a vaporization of toxic isocyanate compound having low boiling point is easily caused.
On the other hand, it has not been easy to form a thin foamable composition layer on a surface, which results a formation of a thin polyurethane foam layer. Heretofore, a method for bonding sheets obtained by slicing a block of a polyurethane foam which is foamed and set, has been widely employed. However, the slicing and bonding of sliced sheets of polyurethane foam are complicated. A method for spread-coating a foamable composition after placing it on a surface has not been easy since the foaming and setting of the foamable composition are speedy. When the position of the moving surface is varied, for example, the surface is a waved surface, it has been difficult to uniformly spread-coat the foamable composition in thin thickness. A method for preparing a thin broad sheet by foaming and setting a foamable composition in a sealed mold has been difficult. For example, when a required quantity of a foamable composition is put in a mold to close the mold and the foamable composition is intended to expand by the blowing pressure of the foamable composition to all corners of the mold in the foaming and setting. The blowing pressure is usually not enough to fill the foamed composition in all of the space.
It is expected to dissolve these difficulties by uniformly spreading the foamable composition in thin thickness, on all of the surface.